Accurately measuring the elevated temperature of a remote object in a very hot environment, such as a a furnace, using optical techniques and without touching the object is complicated by reflections of ambient radiation from the environment along with radiation from the heated object itself. Optical pyrometry allows the temperature of an object to be measured remotely by analyzing the radiation emitted by the object. Of course, all objects at temperatures greater than 0 kelvin emit radiation which can be measured to determine the temperature of the object, provided the emissivity of the object is known. Thus, optical pyrometry operates upon the underlying principle that as the temperature of an object increases, the radiation it emits shifts in wavelength and increases in intensity so that an object which emits radiation with an orange glow is hotter than an otherwise identical object which emits radiation with a red glow. Such temperature-measuring schemes are discussed in the literature (see, for example, Tenney; Mechanical Engineering, Oct. 1986; "Red Hot . . . AND HOTTER," pp. 36-41).
Certain processes for fabricating circuits on silicon wafers require accurate measurement from a remote location of the temperature of a wafer within a processing furnace. In these processes, the emissivity of the wafers may be altered by surface coatings on the wafers of different materials such as silicon dioxide or silicon nitride of different thicknesses, and optical temperature measurements of the wafers must therefore be corrected for emissivity of the wafer to provide accurate optical measurements of the wafer temperature.